1. Field of the Invention
The present invention relates to a displacement measuring apparatus, and in particular to a displacement measuring apparatus, such as a linear encoder or a rotary encoder, that detects displacement information, such as the moving amount, moving speed, and rotation speed, of a moving body using an optical scale.
2. Related Background Art
There has conventionally been used a displacement measuring apparatus that detects displacement information of a moving body using three optical scales.
Such a displacement measuring apparatus is proposed in Japanese Patent Publication No. 60-23282, Japanese Utility Model Application Laid-Open No. 1-180615, and the like, for instance.
There will be described below conventional encoders disclosed in these patent documents that each use three gratings.
First, FIGS. 26 and 27 are each a schematic drawing showing the main portion of an optical system of the displacement measuring apparatus proposed in Japanese Patent Publication No. 60-23282.
In FIG. 26, reference numerals 103, 104, and 105 respectively denote a first scale, a second scale, and a third scale that each have a grating provided with light transmission portions and non-transmission portions at a constant pitch P1, P2, or P3. Also, these first to third scales are each opposedly arranged approximately parallel to a displacement detecting direction 106. Reference numeral 101 indicates a light source that radiates a light flux whose luminescence center wavelength is λm, and reference numeral 102 indicates a light-receiving element. The second scale 104, the third scale 105, the light source 101, and the light-receiving element 102 are integrally contained within a single case. The first scale 103 is provided for a moving body (not shown) and is capable of moving in the direction of an arrow 106.
The light beam from the light source 101 is made incident on the second scale 104, is light-modulated by the second scale 104, and is made incident on the first scale 103. Then, the light beam is light-modulated by the first scale 103, is made incident on the third scale 105, is light-modulated by the third scale 105, and is made incident on the light-receiving element 102 to be detected.
Here, the space between the second scale 104 and the first scale 103 is referred to as “μ”, the space between the first scale 103 and the third scale 105 is referred to as “V”, and a positive number is referred to as “n”, as shown in the drawing. In this case, a geometrical real image concerning the grating of the first scale 103 and a diffraction optical real image concerning the grating of the first scale 103 are respectively formed by the light beam light-modulated by the second scale 104 and the first scale 103 at the position of the third scale 105 if Expressions (1) to (5) given below are satisfied.
(1-a) For geometrical real imageP1/P2=V(μ+V)  (1)P1/P3=μ(μ+V)  (2)1/μ+1/V=λm/(nP12)  (3)
(1-b) For diffraction real imageP1/P2=2V(μ+V)  (4)P1/P3=2μ(μ+V)  (5)
It should be noted that the geometrical real image and the diffraction optical real image will be hereinafter referred to as the “grating image”.
Also, the grating image becomes an image having a periodic contrast with a pitch P3 that is the same as a grating pitch P3 of the third scale 105. In this case, if the first scale 103 moves in the direction of arrow the 106, the grating image forming on the third scale 105 also moves. As a result, the intensity of light passing through the third scale 105 changes along with the movement of the grating image and a periodic displacement signal concerning movement information of the first scale 103 is obtained from the light-receiving element 102. The illustrated displacement measuring apparatus detects movement information of the first scale 103, which is a moving body in this example, using the displacement signal obtained from the light-receiving element 102.
FIG. 27 is a schematic drawing showing the main portion of an optical system of another displacement measuring apparatus proposed in Japanese Patent Publication No. 60-23282 described above. The displacement measuring apparatus in this drawing differs from the displacement measuring apparatus shown in FIG. 26 in that a first scale 113 is of a reflection type and a second scale 116 doubles as a third scale, although there is used the same optical displacement detecting principle.
With the construction shown in FIG. 27, a light beam from a light source 111 is irradiated onto the second scale 116 through a half mirror 117, a light-modulated light beam from the second scale 116 is made incident on the first scale 113, and reflected which is light light-modulated by the first scale 113 is made incident on the second scale 116. Then, the light-modulated light beam from the second scale 116 is detected by a light-receiving element 112 through the half mirror 117.
As described above, there exist two types of displacement detecting systems based on the three-grating-type construction: a transmission-type displacement detecting system and a reflection-type displacement detecting system. However, the reflection-type displacement detecting system has more merits than the transmission-type displacement detecting system. For instance, the number of required scales is substantially reduced from three to two, as can be seen from FIG. 27. In addition, the reflection-type displacement detecting system is favorable in terms of miniaturization, in comparison with the transmission-type displacement detecting system.
The features of the reflection-type displacement detecting apparatus will be described in more detail below with reference to FIG. 27.
With the construction of the reflection-type displacement detecting apparatus, aforementioned Expression (3) “1/μ+1/V=λm/(nP12)” is changed as follows.
In FIG. 27, the second scale and the third scale in FIG. 26 are integrally provided as the scale 116, so that a relation “μ=V” holds true in the above description. Therefore, Expression (3) is changed into Expression (6) given below.V=μ=2n(P1)2/λm (where n=positive number)  (6)
This expression indicates that when the space between the first scale 113 and the second scale (=third scale) 116 is set at “V” described above, an optical fringe pattern having a high contrast is formed on the surface of the third scale. Also, practically, two scales are arranged and used so as to have such a positional relation.
Under the three-grating-type detecting principle, Expression (6) is a general expression that gives a substantial positional relation that is in particular optimum for the reflection-type construction.
Next, there will be described FIG. 28 that is a schematic drawing showing the main portion of an optical system of the displacement measuring apparatus proposed in Japanese Utility Model Application Laid-Open No. 1-180615. In this drawing, reference numerals 123, 124, and 125 (125a, 125b) respectively denote a first scale, a second scale, and a third scale, numeral 121 a light source, and numeral 122 (122a, 122b) a light-receiving element.
The first scale 123 is produced using a reflection-type scale and is provided for a moving body (not shown) so as to be movable in the direction of the arrow 126. The principle of the first scale 123 to detect displacement information is the same as that of the displacement measuring apparatus illustrated in FIG. 26.
That is, a light beam from the light source 121 is made incident on the second scale 124 in a diverging manner, is light-modulated by the second scale, and is made incident on the first scale 123. Then, two reflected lights light-modulated by a displacement of the first scale 123 are detected by the light-receiving elements 122a and 122b through the third scales 125a and 125b provided adjacent to each other on approximately the same plane as the second scale 124. At this time, from the light-receiving elements 122a and 122b, there are obtained displacement signals concerning movement information of the first scale 123 in the direction of the arrow 126, as is the case of the displacement measuring apparatus in FIG. 26.
As the scales shown in FIGS. 26 to 28, there is generally used a metallic scale produced by forming many slit opening portions in a metallic substrate through etching, a glass scale produced by forming many slit opening portions in a glass substrate through evaporation of Chromium thin film or Aluminum thin film or the like and etching, or the like.
In either case of Japanese Patent Publication No. 60-23282 and Japanese Utility Model Application Laid-Open No. 1-180615 described above, a so-called three-grating theory is applied to displacement measurement.
As to the displacement measuring apparatus in FIG. 26 based on such a three-grating theory, there are further disclosed an “array-shaped light-receiving element (photosensitive element array)”, in which the light-receiving element 102 and the third scale 105 are integrally provided, and an “array-shaped light source (light-emitting element array)” in which the light source 101 and the second scale 104 are integrally provided.
FIGS. 29 and 30 are each a schematic drawing showing the main portion of another optical system of the displacement measuring apparatus proposed in Japanese Utility Model Application Laid-Open No. 1-180615. In this patent document, as is the case of Japanese Patent Publication No. 60-23282, there are disclosed an “array-shaped light-receiving element 232 (photosensitive element array)” shown in FIG. 29 and an “array-shaped light source 230 (light-emitting element array)” shown in FIG. 30.
Still further, as other systems of the reflection type, there are proposed a construction shown in FIG. 32 where a shortage of light intensity is compensated for using a lens and a construction shown in FIGS. 33 and 34 where direct reflected light is used as a reflecting body and a distance between a reflecting body and a light source is further reduced, thereby avoiding the shortage of light intensity.
In the formerly described several prior arts proposing a system of the reflection type based on the three-grating principle and in the latterly described prior arts proposing a system in which there is made arrangement so that a lens is provided or the distance between a light source and a scale is reduced, however, there exists the following drawbacks.
In the reflection-type displacement measuring apparatus shown in FIG. 27, there is used a half mirror, so that the size of the apparatus is increased as a whole and the loss of intensity of light from the light source is considerably increased due to the existence of the half mirror. Accordingly, it is required to increase the intensity of light emitted from the light source, which results in an increase in power consumption.
In the displacement measuring apparatus shown in FIG. 28, light emitted in a light source optical axis direction (optical axis A of the light source in FIG. 28) has high intensity, but is not made incident on the light-receiving element and is reflected in a light source direction. This leads to a situation where the light does not become a substantially effective light beam, which also results in an increase in power consumption in a like manner.
Further, in the case of a displacement measuring apparatus functioning as an encoder or the like, it is generally required to obtain displacement signals in a plurality of different phases in order to detect a displacement direction as displacement information of a moving body.
In FIG. 28, in a like manner, in order to obtain a plurality of displacement signals having different phases, it is required to perform layout of various components such as the components 125b, 125a, 122b, and 122a in the illustrated manner. FIG. 31 shows a concrete arrangement of these components. In this drawing, there is illustrated a construction where light emitted from a light source 42 reaches a main scale 340 through an index scale 342, is reflected by the main scale 340, passes through the index scale 342 again, and reaches a light-receiving element 348.
In either case of the displacement measuring apparatuses shown in FIGS. 26, 27, and 28, in order to obtain a plurality of displacement signals having different phases, it is required to classify gratings of scales with reference to the number of required signals, perform layout of the gratings so as to have them displaced from each other by a predetermined phase difference, and provide a plurality of light-receiving elements in order to obtain respective signals. As a result, the apparatus becomes complicated as a whole. In addition, the sizes of the gratings (scales) and the total size of the apparatus are increased.
With the construction shown in FIG. 32 in which a lens is provided, there is obtained a result that is favorable in terms of the light intensity shortage problem but unfavorable in terms of miniaturization. If the distance between the lens and the light source is shortened and the lens curvature is reduced in order to realize miniaturization and thickness reduction, it is required to increase the accuracy of optical axis matching, which means that there are imposed limitations on size and thickness reduction. In order to enhance the accuracy and resolution, a higher-accurate lens performance is required, which hinders the realization of cost reduction.
In the case of the construction shown in FIGS. 33 and 34, in order to obtain a practical construction, it is required to cover the electric elements, such as the light-emitting element and the light-receiving element, with a protecting member, so that an adjacent arrangement is impossible to an extreme extent. Also, as can be seen from these drawings, a light beam on the optical axis of the light source is not made incident on the light-receiving portion, so that it is impossible to say that there is obtained sufficient light usage efficiency. In particular, this construction departs from the three-grating-type detecting principle to some extent, and is the equivalent of a construction where not a plurality of slits but only one slit is established in the optical scale arranged on the light source side. In this case, it is preferable that the size of the light-emitting region of the light source in the displacement direction of a reflection scale is set at around ½ of the pitch of a reflecting slit when the size setting is performed in compliance with the three-grating-type principle. As a result, it becomes necessary to use a light-emitting device that emits light within a minute window. If a current injected into the light source element is increased in order to obtain required intensity of received light with this construction, the current density is increased because the light-emitting region is minute, so that there may occur breakage of the elements. As a result, there is no choice but to limit the injection current.
As described above, in the reflection-type encoder in compliance with the three-grating-type principle, if a half mirror or the like is used as a reflecting means/construction, the apparatus size is increased, which has lead to a difficulty in realizing miniaturization up to now. Also, if there is made an attempt to realize miniaturization, it becomes impossible to effectively use on-axis rays of a light source, as can be seen from FIGS. 28, 31, 33, and 34. As a result, there occurs a shortage of light intensity.
In addition, the usage of a lens is conceivable as a measure to increase the light intensity, although this construction departs from the originally intended three-grating-type detecting principle and there occurs a problem concerning a position detecting function.